1. Field of the Invention
The present invention relates to a process of manufacturing an ink jet print head. More in particular, the present invention relates to process of manufacturing an ink jet print head comprising a silicon nozzle plate.
2. Description of Related Art
The present invention relates to a method of manufacturing an ink jet print head and in particular a print head of the type in which droplets of ink are expelled from a nozzle by rapid heating of a resistive element contained within an ink collecting chamber and disposed next to the nozzle.
The ink collecting chamber and the resistive element are formed within a multi-layer structure manufactured on a silicon substrate using well known methods of manufacturing of semiconductor devices.
In short, various layers are deposited on a face of the silicon substrate to make up the ejection resistors and, possibly, the active electronic components. After that, a barrier layer of photopolymer is coated on the silicon substrate. Using photolithographic techniques, the ejection cells and ink delivery channels are made in the barrier layer photopolymer and a nozzle plate provided with ejection nozzles built in correspondence with the cells is mounted.
The nozzle plate is an essential component of an ink jet print head. Some known methods of forming ink jet nozzle plates use one or more intermediate molds. One of such methods uses an electroforming process. The electroforming process uses a mold (or mandrel) overcoated with a continuous conductive film having non-conductive structures that protrude over the conductive film. A metallic ink jet nozzle plate is formed using such a mold (or mandrel) by electroplating onto the conductive film. Over time, the metallic layer grows in thickness. The ink nozzles are defined by the non-conductive structures. The metal employed for electroforming nozzle plate is usually nickel coated with gold or palladium.
The metallic nozzle plate is usually bonded to photopolymer layer of the ink jet print head by applying a pressure, e.g., of from 1 to 5 bars, at relatively high temperature, typically ranging from 150° to 200° C., for a certain period of time, e.g., from 1 to 2 hours, this process being commonly known as thermo-compression bonding.
However, the Applicant has noticed that the thermal bonding between the metallic nozzle plate and the photopolymer layer coated on the silicon substrate creates mechanical forces, due to the different coefficient of thermal expansion of the materials, which can cause the rupture of the resulting print head either during the thermo-compression bonding or during the subsequent manufacturing process steps. The different thermal expansion of the materials is a significant issue in the prior art, which limits the size of ink jet printheads that can be manufactured. This is because the difference in the coefficient of thermal expansion between, for example, a nickel nozzle plate and a substrate to which the nozzle plate is connected, where this substrate is of silicon, is quite substantial. Consequently, along a distance occupied by, for example, 1000 nozzles, the relative thermal expansion that occurs between the respective parts when heated from the room temperature to the temperature required for bonding the metallic nozzle plate and the photopolymer layer together can cause a dimension mismatch even greater than a whole nozzle length. This would be significantly detrimental for such devices.
Recently, there has been an increasing interest in manufacturing the nozzle plate starting from a silicon substrate.
U.S. Pat. No. 6,863,375 relates to a method of forming a nozzle plate by etching a silicon monocrystalline substrate. The piezoelectric ink jet head is then arranged by bonding together the nozzle plate and the cavity plate, both made of silicon monocrystalline, with the glass substrate. The nozzle plate disclosed in this specification has a thickness of 180 μm.
U.S. Pat. No. 6,303,042 discloses a method for forming an ink jet nozzle plate using a SOI wafer having a top substrate layer, a buried layer, and a bottom substrate layer. The method substantially comprises the steps of (1) providing a composite mask over the top substrate layer having a cavity mask which provides openings and a bore mask having openings which are entirely within the openings of the cavity mask and extend to the top substrate layer, (2) anisotropically etching through the bore mask openings through top substrate layer and the buried layer into a portion of the bottom substrate layer, (3) removing the bore mask and etching the top and bottom substrate layers without substantially affecting the buried layer to extend the openings in the top substrate layer and the bottom substrate layer, (4) removing the cavity mask and attaching the top substrate layer to a base provided with ink delivery channels with correspond to the openings in the buried layer, (5) and removing the bottom substrate layer. The removal of the bottom substrate layer is made by wet or dry etching or, preferably, by mechanical grinding followed by chemical polishing or by plasma etching.
U.S. Pat. No. 6,673,694 discloses a general fabrication method for producing MicroElectroMechanical Systems (MEMS) and related devices using Silicon-On-Insulator (SOI) wafers. A SOI wafer has (i) a handle layer, (ii) a dielectric layer, and (iii) a device layer. The SOI wafer is patterned and etched in the device and dielectric layers and then bonded to a substrate of glass, silicon, or other material. Then the handle layer of the SOI wafer is removed by wet or dry etching, followed by the removal of the dielectric layer of the SOI wafer.
Recently, the use of debondable SOI wafers has been theorized for applications where a thin layer needs to be applied to a substrate such as, for example, in the field of microelectromechanical systems. In debondable SOI wafers, comprising (i) a handle layer, (ii) a debonding layer, and (iii) a device layer, the handle layer is removed from the device layer by mechanical debonding action (H. Moriceau, O. Rayssac, B. Aspar, B Ghyselen, “The Bonding Energy Control: An Original Way To Debondable Substrates”, Presented at ElectroChemical Society Conf. Paris May 2003—Wafer-Bonding Symposium (said article have been downloaded on Oct. 31, 2006 from the website of Tracit Technologies, a French company having its headquarters in Moirans, France, at http://www.tracit-tech.com/Publi/Publi—2.pdf). Debondable SOI wafers and method of manufacturing thereof have been also disclosed, for example, in International Patent Application WO2005/034218.
The Applicant has noticed that the current tendency to the reduction of the volume of the drop ejected by the nozzle plate, now in the order of a few picoliters, is not compatible with a nozzle plate having a thickness higher than 100 μm, and even less higher than 150 μm. Rather, the thickness of the current nozzle plates for thermal ink jet print head is in the range of from 20 to 100 μm. Silicon wafers having such a thickness are expensive and are not usually commercialized. Further, silicon wafers having a thickness of from 20 to 100 μm would be very difficulty handled and worked.
On the other hand, the Applicant has noticed that the removal of the handle or bottom layer of a conventional SOI wafer by etching or mechanical grinding, though possible, has many drawbacks. First, the process of removing several hundreds of micrometers of silicon is a long and expensive operation. Second, there could be some incompatibilities between the removal process and the printhead. Third, the mechanical grinding removal can cause damages to other components of the printhead.
Finally, the Applicant has found that when a method employing a debondable SOI wafer is applied to the manufacturing of ink-jet print head, wherein the silicon nozzle plate to be attached to a barrier layer could be obtained from the device layer, the following additional problems arise. If the device layer of the SOI wafer is etched to create the nozzles after the bonding of the SOI wafer to the substrate, the actuators can be damaged during the etching process. On the other hand, if the device layer of the SOI wafer is etched to create the nozzles before bonding the SOI wafer to the barrier layer, the structure of the SOI wafer is weakened and the device layer can be damaged during the step of debonding of the handle layer.
Accordingly, there is still the need to an improved method for overcoming the above mentioned problems.
The Applicant has found that the aforementioned problems can be avoided by using a process for manufacturing an ink-jet print head comprising the steps of                a) providing a print head wafer comprising a plurality of print head dice coated with a barrier layer, each print head die comprising a plurality of actuators and interconnections, said barrier layer comprising a plurality of openings in correspondence with said plurality of actuators and interconnections,        b) providing a debondable SOI wafer comprising a handle layer, a buried layer, and a device layer,        c) forming a protective layer on the surface of said device layer,        d) bonding said device layer to said barrier layer,        e) debonding said handle layer from said SOI wafer,        f) etching said device layer so as to realize a plurality of openings in correspondence with said plurality of actuators and interconnections, and        g) removing said protective layer in correspondence of said plurality of openings.        
The Applicant has found that the process of the present invention allows to realize inkjet print head comprising silicon nozzle plate having a thickness lower than 100 μm without using expensive thin silicon layers and complex handling and manufacturing operations.
Moreover, the Applicant has found that the process of the present invention allows to easily remove the handle layer without causing damages to the printhead.
Finally, the Applicant has found that the process of the present invention allows to avoid damages to the actuators of the inkjet printhead without weakening the structure of the SOI wafer.